Decreasing landfill space and rapidly rising disposal costs have forced many municipalities to begin curbside recycling of post-consumer plastic (polymeric) waste. In general, plastic materials comprise approximately 20% by volume of the municipal waste stream. For example, Chem Systems, 1992, reports that municipal solid waste comprises, by weight, 48% polyethylene (PE) (27% being low density PE and 21% being high density PE), 16% polypropylene (PP), 16% polystyrene (PS), 6.5% polyvinyl chloride (PVC), 5% polyethylene terephthalate (PET), 5% polyurethane, and 3.5% other plastics.
Post-consumer polymeric waste, as opposed to industrial plastic waste, typically includes substantial quantities of plastic bottles, containers and packaging materials. Plastic bottles are molded of different polymeric materials depending upon the product they are to contain. For example, plastic bottles for water, milk, and household chemicals typically are made of high density polyethylene (HDPE), while soft drink bottles are typically made of polyethylene terephthalate (PET) with or without base caps made from high density polyethylene (HDPE). Generally, HDPE bottles account for approximately 50-60% and PET bottles account for approximately 20-30% of the bottles used by consumers.
The balance of bottles, bottle caps and other containers used by consumers comprises other polymeric materials, such as low density polyethylene (LDPE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), and other resins and multi-layered materials.
Plastic packaging materials also are made of a wide variety of polymers. For example, according to Plastics Compounding, November/December, 1992, the following polymers were used in packaging material in the %'s set forth: 27% LDPE, 21% HDPE, 16% PS, 16% PP, and 5% PET.
Post-industrial plastic waste can comprise polyolefins, PS, PET and other polymeric materials used for plastic packaging.
Currently, collection of plastic waste material exceeds the market demand for recycled plastic products as a result of the dearth of viable recycling technologies that are low cost and produce high quality recycled plastic products. One recycling approach has involved the high energy consuming batch grinding of commingled, unsorted mixed color plastic waste to form flake scrap material, melt processing and pelletizing the melt processed material to pellets, and extruding the pelletized plastic waste to form recycled plastic products. However, recycled plastic products made in this manner suffer from severe deficiencies that render the products unsatisfactory for many purposes and are of inferior, low value compared to products made of virgin polymeric materials. For example, these recycled plastic products exhibit inferior mechanical properties (e.g. tensile, flexural and impact strength) and inferior appearance in terms of color (dark brown or gray color) with streaking of colors within the molded product as a result of the chemical incompatibility of the different polymers present in the initial plastic waste stream and variations in the plastic waste stream composition over time.
A typical example of a low value, recycled plastic product is recycled plastic lumber having a dark brown or gray color with noticeable color streaking and inferior mechanical properties compared to components molded of virgin materials. As a result of the less than pleasing appearance, recycled plastic lumber is oftentimes painted to improve its appeal to the customer, or expensive pigments and other additives are added to the feedstock during the manufacturing process to this end. However, the cost of the recycled product is increased thereby.
Furthermore, certain melt processing techniques, such as blow molding, rotational molding, extrusion (e.g. extruded PVC pipe and profiles), and spray coating, require a plastic powder feedstock. That is, the flake scrap material is not directly melt processable to articles of manufacture by such powder feedstock-requiring melt processing techniques. To be useful as feedstock in such melt processing techniques, sorted or unsorted flake scrap material produced by batch grinding must be pelletized and then ground to powder form. The need to pelletize and grind sorted or unsorted flake scrap polymeric material prior to such melt processing adds considerably to the cost and complexity of recycling scrap plastics as well as the capital equipment expenditures required.
Currently used injection molding techniques require plastic pellets for high speed production of molded parts. Although unsorted, commingled flake scrap materials could be pelletized to provide feedstock for injection molding, the resultant molded products would suffer from the types of deficiencies discussed above attributable to polymer incompatibility.
So-called compatibilizing agents and/or reinforcing agents can be added to flake plastic scrap material comprising chemically incompatible polymers in attempts to produce a recycled plastic product exhibiting more desirable characteristics. However, addition of these agents to the plastic scrap material makes recycling more difficult and adds considerably to its cost. The Mavel et al. U.S. Pat. No. 4,250,222 relates to this type of recycling approach and is representative of the disadvantages associated with such an approach to plastic recycling.
Attempts have been made to sort commingled, post-consumer plastic scrap to overcome the polymer incompatibility problems associated with the recycling of commingled plastic scrap. To-date, HDPE and PET are recovered from plastic waste streams by recycling technologies requiring sorting of the commingled plastic materials. Sorting can require use of costly techniques, such as video cameras, electronic devices, infrared detectors, and organic "markers", to provide effective segregation of like plastics. However, even sorted plastic waste can present problems in processing as a result of density and chemical differences among polymers falling in the same general class and made by different plastics manufacturers.
Further, sorted plastic scrap must be subjected to batch grinding to produce flake scrap material that then must be pelletized and ground again to provide powder feedstock for blow molding, rotational molding, some extruding, spray coating and other melt processing techniques that require powder feedstock.
The high cost of sorting has greatly limited widespread use of recycling approaches that require a sorting step. In particular, collected and sorted post-consumer plastic materials are usually more expensive than the corresponding virgin plastic materials. Thus, users of plastic materials are discouraged from using sorted, recycled plastic materials.
It is an object of the invention to provide a method of processing one or more polymeric materials, such as sorted or unsorted, commingled scrap polymeric material, by solid state pulverization to produce pulverized particulates (e.g. powder) that can be directly formed to shape by powder feedstock-using melt processing techniques.
It is another object of the invention to provide a method of processing polymeric materials, such as sorted or unsorted, commingled scrap polymeric materials, having mixed colors by solid state pulverization to produce pulverized particulates that are melt processable to a substantially homogeneous light color without color streaking or marbleizing despite being produced from the mixed color scrap materials.
It is a further object of the invention to provide a method of processing polymeric materials in a manner to achieve in-situ compatibilization of different polymers present.
It is a further object of the present invention to provide a method of recycling commingled scrap polymeric materials without sortation and in a manner to achieve in-situ compatibilization of different polymers present and produce recycled polymeric particulates without the need for a compatibilizing agent.
It is still another object of the present invention to provide a method of recycling commingled, mixed-color scrap polymeric materials without sortation and in a manner to produce recycled polymeric particulates that are melt processable to homogeneous light color without color streaking or marbleizing.
It is still another object of the present invention to provide solid state pulverized polymeric particulates that are suitable as powder feedstock for melt processing by blow molding, rotational molding, some extruding, spray coating and other powder feedstock-using melt processing techniques.
It is still a further object of the present invention to provide solid state pulverized polymeric particulates that are melt processable to a homogenous light color, despite being produced from mixed-color polymers.
It is still a further object of the invention to produce articles of manufacture, including molded parts and coatings, made from the aforementioned solid state pulverized polymeric particulates.